Wireless communication receivers, particularly digital wireless communication receivers, must generally be synchronized in time and frequency with a particular received to communication signal in order to be able to correctly parse the particular signal from other signals or noise throughout the radio frequency (RF) spectrum. For example, for any given (e.g., expected) time and frequency, due to propagation delays, frequency shifts, multipath deflection, etc., as will be appreciated by those skilled in the art, a receiving device often needs to align (“tune”) its acquisition of the transmitted signal to account for these various offsets from the expected results.
Accordingly, in many communication systems, at the start of each communication, a preamble or “acquisition sequence” may be transmitted from a transmitting device such that any interested receiver may detect the transmission and synchronize to it. For instance, these acquisition sequences may be generally known communication signal structures that facilitate waveform acquisition processing through unique timing and frequency properties. The remainder of the transmitted data (e.g., the payload) may then be transmitted and received based on the synchronization, with optional “pilot” sequences dispersed throughout the payload to ensure that the transmitter and receiver remain synchronized (e.g., due to environmental changes, movement of the devices, etc.).
For certain radio communications, transmit diversity may be used to improve communication performance. For instance, multiple-input and single-output (MISO) technology is the use of multiple antennas/transmitters at a transmitting device and a single antenna/receiver at a receiving device, while multiple-input and multiple-output (MIMO) technology involves the use of multiple antennas at both the transmitter and receiver. Transmission diversity is particularly useful in wireless communications because it offers significant increases in data throughput and link range without additional bandwidth or transmit power. Specifically, this may be achieved by higher spectral efficiency (more bits per second per hertz of bandwidth) and link reliability or diversity (reduced fading). In other words, MIMO and MISO systems are multi-transmitter communications systems where typically all transmitters (e.g., 2, 3, 4, etc.) transmit signals simultaneously on the same bandwidth (at the same time and on the same frequencies). Note that only some of all of the signals may be received, e.g., due to fading channels, hence the usefulness of MIMO and MISO, where there may be multiple different possibilities to communicate (e.g., a “4×4” MIMO with 4 transmitters×4 receivers=16 communication possibilities).
Receivers in a transmit diversity scheme generally need to acquire each of the multiple transmitters separately in order to understand the overlapping transmitted signals. That is, the receivers are unable to de-conflict/recover the multiple signals until synchronizing to each of the separate transmitters. The acquisition sequence (preamble) mentioned above, however, is typically useful only for single antenna transmission, and not for transmit diversity. For instance, in systems which transmit from multiple antennas to provide transmit diversity, synchronization signals from the multiple antennas would be superposed on a single receive antenna, and thus would generally interfere with one another.
Various solutions have been utilized to allow receivers in a transmit diversity architecture to acquire (synchronize to) the multiple transmitters, but each has had its own drawbacks. For example, in one approach, the synchronization structure may be transmitted from only a single antenna, such that receivers may use a single correlator to detect the structure. This approach, however, provides no diversity benefit, possibly allowing for the synchronization structure to be lost (e.g., faded), and also wastes the power from one or more transmitters. In another approach, a first antenna may transmit the sequence, and when complete, a second antenna may transmit a sequence, etc., such that each antenna's (transmitter's) sequence does not overlap with another antenna's. This, unfortunately, multiplies the time required to acquire each antenna with the number of antennas, thus doubling for two antennas, tripling for three antennas, etc. Alternative approaches require computationally expensive algorithms, such as transmitting distinct pseudonoise structures from each antenna, and using multiple correlators to detect the superposed structures. This computationally expensive approach allows the synchronization structures to interfere, but incurs a resulting performance degradation. Also, an even more computationally expensive approach involves transmitting orthogonal synchronization structures from multiple antennas, such as orthogonal acquisition waveforms for orthogonal frequency division multiplexing (OFDM), and using multiple correlators to detect the structures. This approach, while providing superior performance, is the most computationally expensive approach.
Acquiring a signal (receiver synchronization) has thus been the weakest link for transmit diversity technologies, particularly without providing acquisition (synchronization) processing circuitry that is vastly more complex than the circuitry required to demodulate the synchronized data signal itself.